The present invention relates to stray voltage reduction apparatus for use in supplying electrical power to a service entrance to reduce stray voltages. More specifically, the present invention relates to improved stray voltage reactors and combinations of such improved stray voltage reactors with other components.
Stray voltages or neutral-to-earth (NE) voltages are an ongoing problem in many areas supplied from distribution and where current levels on the grounding systems are above the perception threshold for either animals or people. Typical locations are dairy farms, feeder and confinement operations, swimming pools, water systems, and residences. For instance, on the dairy farm, small electrical currents passing through dairy cows can cause aggravation and even mastitis, which results in milk losses and poor animal health.
Stray voltages have many sources, both from on and off the site of electrical power consumption. On-site stray voltage can be resolved through a program of upgrading and reconnecting the wiring system and its various loads. Off-site stray voltage can result from primary neutral currents, off-site faults, and marginal groundings. Steps to correct the stray voltage problems may be delayed or not undertaken at all if the power utility company believes that the problems are originating on-site and the on-site owner believes that the problems are originating with the utility off-site.
If the stray voltage is associated with the primary neutral of a distribution transformer of the utility, an isolation transformer can be installed by the owner on the site to eliminate primary neutral currents. However, the KVA (kilovoltampere) rating of the isolation transformer must be on the order of 25 KVA to isolate whole dairy barns, for example. The cost of the isolation transformer approach to stray voltage reduction is consequently quite high.
Another approach involves using an electronic circuit to sense unbalance in the system and inject cancelling voltages into a separate ground system. The cost is also relatively high and maintenance and safety considerations can become important.
"Presently Used Treatments or Corrective Procedures for Stray Voltage Problems" by T. C. Surbrook, Workshop On Stray Voltages in Agriculture, Aug. 10-11, 1983, Minneapolis, suggests in connection with FIG. 4 therein that a saturable-core reactor may be installed between the primary and secondary neutrals at the power supplier's transformer to obtain stray voltage isolation. The saturable-core reactor is there stated to be a device that offers a resistance to the flow of neutral-to-earth currents and develops a magnetically saturated core upon fault current conditions. With the core saturated, the resistance of the reactor is said to drop very low. The therestated principle of operation of the saturable core reactor is that it would offer adequate resistance to the flow of neutral current across the primary to secondary transformer bonding strap so that most of the neutral-to-earth voltage occurs across the reactor. This leaves little voltage across the resistance of ground rods and equipment to earth.
The voltage level at which saturation occurs is termed the "saturation point" herein. The saturating reactor must have a saturation point which exceeds the stray voltages which can occur in the absence of a fault, but which is less than the voltage which will be developed at the primary neutral when a fault occurs.
The theory of the saturating reactor approach, as set forth in the above-cited paper, assumes that a sufficiently high resistance can be obtained to reduce the stray voltage below the perception thresholds of animals and people with a reactor also having a practical saturation point and current rating. In actuality, the resistance requirement and the saturation point requirements conflict. As a practical matter, many agricultural areas have a relatively high ground resistance which means that a very high resistance of the saturating reactor is required but cannot be obtained because a practical saturation point and current rating are also needed. Saturating reactors of even a fraction of the required stray voltage reducing characteristics are so bulky and expensive and inconvenient to install that they would be impractical.
Furthermore, a reactor actually presents an AC impedance, and not a resistance, strictly speaking. Consequently, the reactor actually causes a reactive voltage drop, which is 90.degree. out of phase with the voltage resistively developed across the ground impedance. As a result, the resistively developed voltage across the ground impedance is not reduced as much as it would be if the reactor caused a resistive voltage drop. The reactive voltage drop in currently known stray voltage reactors would not adequately reduce the stray voltage to which the animals are exposed when the ground resistance is as high as it is in many localities.
In view of the above considerations, the saturating reactor approach appears to be generally impractical. Unless a manufacturer is prepared to consider foregoing marketing in areas having high ground impedances and to have the ground impedance determined at each purchaser's site to be sure the saturating reactor will perform the function intended for it, there is little reason to make stray voltage reactors commercially available except as a low volume, high cost item.